Geological interpretation of a deep seismic reflection profile across the Eastern Province and Median Batholith, New Zealand: Crustal architecture of an extended Phanerozoic convergent orogen

Abstract

An offshore seismic reflection profile provides new information on the tectonic evolution of the Paleozoic‐Mesozoic margin of southern Gondwanaland. The 550 km long composite South East South Island (SESI) profile extends from near Christchurch to near Stewart Island, New Zealand. It crosses the Eastern Province geological basement terranes and ends in the Median Batholith.

The Median Batholith and all the basement terranes appear to extend down to middle or lower crustal levels and are not minor thin‐skinned fault slices. A discontinuous, but generally strong Moho reflector is observed along the profile varying from c. 22 to 32 km depth. Upper to mid‐crustal reflections can be related to Murihiku Terrane bedding and Otago Schist foliation. Gravity and magnetic measurements indicate that the Maitai Terrane dips northeast beneath the Otago Schist. The SESI profile contains no evidence for stranded slabs of Mesozoic oceanic crust beneath the Eastern Province.

The Moho depth along the SESI profile, particularly under the Median Batholith and Otago Schist accretionary prism, is markedly shallower than in other circum‐Pacific Mesozoic orogens. This, and and the geometry of several fault‐related reflectors in the SESI profile, can be attributed to significant out‐of‐profile Late Cretaceous extension related to Gondwanaland breakup. Three regions of strong, deep crustal reflectivity are present, and their association with thinner crust suggests they may also be extension‐related features.

Keywords:

• reflection seismology
• SESI profile
• crustal structure
• tectonics
• South Island
• New Zealand
• Moho
• Eastern Province terranes
• Median Batholith
• Paleozoic
• Mesozoic